Twin-twin transfusion syndrome (TTTS) is one of the most serious complications of monochorionic multiple gestations. The cardinal prenatal findings are monochorionic placentation with concordant gender and discordant amniotic fluid volumes (maximum vertical pocket: donor <2 cm and recipient >8 cm ). TTTS is associated with a high risk of fetal/neonatal mortality, especially in previable gestations, and fetuses who survive are at risk of severe cardiac, neurologic, and developmental disorders.

Twin anemia polycythemia sequence (TAPS) is an atypical chronic form of TTTS without oligohydramnios-polyhydramnios sequence. Prenatally, TAPS can be diagnosed when the middle cerebral artery-peak systolic velocity (MCA-PSV) is greater than 1.5 multiples of median (MoM) in one twin and less than 0.8 MoM in the other twin, although criteria are not uniform across studies. Placental discordance is typically noted on ultrasound: the anemic donor has a thickened hyperechoic placenta and the plethoric recipient has a thinner hypoechoic placenta, with clear demarcation between the donor and recipient territories.

Incidence — The incidence of TTTS is not clear since some fetal losses in monochorionic multifetal gestations in the first half of pregnancy may be related to undiagnosed TTTS [1]. For this reason, incidence based on data from live borns or sonograms in the second half of pregnancy may not be accurate. Our best estimate of the incidence of TTTS is 1:40 to 1:60 twin pregnancies [2-5], 9 to 15 percent of monochorionic diamniotic pregnancies [1,3], and 6 percent of monoamniotic pregnancies [6]. In one report, monochorionic diamniotic twins conceived by in vitro fertilization had a lower incidence of TTTS than those conceived naturally (1/43 [2 percent] versus 36/284 [13 percent]) [7].

Pathophysiology — There is no animal model for study of TTTS, but computer models have been developed [8-10].

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